1. Technical Field
The present invention relates to a semiconductor memory apparatus. In particular, the present invention relates to a clock generator for a semiconductor memory apparatus.
2. Related Art
In general, a DLL (Delay Locked Loop) is a clock generator compensating for a skew between an external clock and data or between an external clock and an internal clock.
A clock generator according to the related art includes a clock receiver 10, a delay element array 11, a buffer 12, a phase separator 13, a replica delay element 14, a phase comparator 15, and a shift register 16, as shown in FIG. 1.
The clock receiver 10 receives external clock eCLK and inverted external clock eCLKb having phase opposite to the external clock and outputs an internal clock iCLK.
The delay element array 11 is composed of a plurality of delay elements. The delay element array 11 delays the internal clock iCLK by a delay time of a delay element selected according to the external control and outputs the delayed internal clock.
The buffer 12 buffers and outputs the output of the delay element array 11.
The phase separator 13 separates the output of the buffer 12 into clocks RCLK and FCLK that have a phase difference of 180° therebetween.
The replica delay element 14 is a delay element modeled on the time required to read out data from a corresponding semiconductor memory apparatus. The replica delay element 14 delays the clock output from the delay element array 11 by a predetermined amount of time and outputs the delayed clock fbCLK.
The phase comparator 15 compares the phase of the internal clock iCLK output from the clock receiver 10 with the phase of the clock output fbCLK from the replica delay element 14 and outputs an up signal UP or a down signal DN to coincide the phase of the iCLK with the phase of the fbCLK.
When the shift register 16 receives the up signal, the shift register 16 causes a delay element having a longer delay time than the previous delay element to be selected from the delay element array 11. When the shift register 16 receives the down signal, the shift register 16 causes a delay element having a shorter delay time than the previous delay element to be selected from the delay element array 11.
Then, the delay element array 11 delays the internal clock iCLK by the delay time of the delay element selected by the shift register 16, and outputs the delayed internal clock iCLK.
As described above, the phase comparison process and the delay time adjusting process through the delay element array 11, the replica delay element 14, the phase comparator 15, and the shift register 16 are repeated such that the buffer 12 outputs a phase-fixed clock DLL_CLK, thereby compensating for the skew between the external clock eCLK and the internal clock iCLK.
In the clock generator for a semiconductor memory apparatus according to the related art, a clock period becomes shorter as the speed of the operation of the semiconductor memory apparatus becomes higher, which causes a problem in that it is difficult to correct the phase of the clock, and thus a clock generating error frequently occurs. In addition, the related art has a problem in that the reliability of the clock is lowered due to the frequent clock errors and a system using the clock generator may operate incorrectly.